The present invention relates, generally, to systems for polishing or planarizing workpieces such as semiconductor wafers. More particularly, the present invention relates to an apparatus for improving the uniform polishing of workpieces where the apparatus comprises a workpiece retaining element that engages a workpiece against a polishing surface during a polishing procedure.
Many electronic and computer-related products such as semiconductors, CD-ROMs, and computer hard disks, require highly polished surfaces in order to achieve optimum operational characteristics. For example, high-quality and extremely precise wafer surfaces are often needed during the production of semiconductor-based integrated circuits. During the fabrication process, the wafers generally undergo multiple masking, etching and dielectric and conductor deposition processes. Because of the high-precision required in the production of these integrated circuits, an extremely flat surface is generally needed on at least one side of the semiconductor wafer to ensure proper accuracy and performance of the microelectronic structures created on the wafer surface. As the size of integrated circuits decreases and the density of microstructures on integrated circuits increases, the need for accurate and precise wafer surface polishing increases.
Chemical Mechanical Polishing (xe2x80x9cCMPxe2x80x9d) machines have been developed to. polish or planarize semiconductor wafer surfaces to the flat condition desired for integrated circuit components and the like. For examples of conventional CMP processes and machines, see U.S. Pat. No. 4,805,348, issued Feb. 21, 1989 to Arai et al.; U.S. Pat. No. 4,811,522, issued Mar. 14, 1989 to Gill; U.S. Pat. No. 05,099,614, issued Mar. 31, 1992 to Arai et al.; U.S. Pat. No. 5,329,732, issued Jul. 19, 1994 to Karlsrud et al.; U.S. Pat. No. 5,498,196, issued Mar. 12, 1996 to Karlsrud et al.; U.S. Pat. No. 5,498,199, issued Mar. 12, 1996 to Karisrud et al.; U.S. Pat. No. 5,558,568, issued Sep. 24, 1996 to Talieh et al.; and U.S. Pat. No. 5,584,751, issued Dec. 17, 1996 to Kobayashi et al.
Typically, a CMP machine includes a wafer carrier configured to hold, rotate, and transport a wafer during the process of polishing or planarizing the wafer. The wafer carrier is rotated to cause relative lateral motion between the polishing surface and the wafer to produce a substantially uniform thickness. In general, the polishing surface includes a horizontal polishing pad that has an exposed abrasive surface of cerium oxide, aluminum oxide, fumed/precipitated silica, or other particulate abrasives. Commercially available polishing pads may utilize various materials, as is known in the art. Typically, polishing pads may be formed from a blown polyurethane, such as the IC and GS series of polishing pads available from Rodel Products Corporation in Phoenix, Arizona. The hardness and density of the polishing pad depends on the material that is to be polished and the degree of precision required in the polishing process.
During a polishing operation, a pressure element (e.g., a rigid plate, a bladder assembly, or the like), which may be integral to the wafer carrier, applies pressure such that the wafer engages the polishing surface with a desired amount of force. The carrier and the polishing pad are rotated, typically at different rotational velocities, to cause relative lateral motion between the polishing pad and the wafer to promote uniform polishing. Most conventional carrier assemblies include some form of retaining structure that maintains the position of the wafer under the pressure element during polishing. Prior art carrier assemblies designed for compatibility with circular wafers employ round retaining structures such as retaining rings.
Retaining rings may either be fixed or xe2x80x9cfloatingxe2x80x9d within the wafer carrier. For example, U.S. Pat. No. 5,695,392, issued Dec. 9, 1997 to Kim, discloses the use of a fixed retaining ring collar that is bolted to the main carrier housing. U.S. Pat. No. 5584,751, issued Dec. 17, 1996 to Kobayashi et al., and U.S. Pat. No. 5,795,215, issued Aug. 18, 1998 to Guthrie et al., each teach the use of a floating retaining ring and a pressure regulating mechanism that controls the biasing pressure applied to the retaining ring.
Typically, retaining rings are made from engineering polymers such as, for example, acetal homopolymer, acetal copolymer, and polyphenylene sulfide. These materials are prone to wear due to the friction between the wafer, polishing pad and slurry abrasives that are used during polishing of the wafer. Wearing of the materials that comprise the retaining rings results in shortening the lives of the retaining rings which are functional and necessary components of the wafer carriers. Water absorption by the retaining rings can also distort dimensions of the acetal copolymers or homopolymers which comprise the retaining rings, thereby distorting the dimensions of the retaining rings themselves. Downtime associated with the repair or replacement of wafer retaining elements such as, for example, retaining rings, used in wafer carriers may be extremely undesirable, particularly if the workpiece throughput is critical.
An alternative to polymer retaining rings are retaining rings made of ceramic materials that are better able to withstand wear from friction created between the wafer and the retaining element, as well as abrasive slurries such as silicon dioxide and aluminum oxide. Because direct contact between the wafer and the retaining ring may result in damage to the wafer, prior art retaining rings may include a disposable liner positioned around the inside diameter of the retaining ring. The disposable liner is typically comprised of a material that is softer than the semiconductor wafer, for example, a polymer such as acetal copolymer or polybutyline terathalate (PBT) to prevent the retaining ring from damaging the wafer.
Despite their resistance to wear compared to retaining rings made from engineering polymers, ceramic retaining rings are still subject to chipping, cracking and other wear effects due to the friction between the wafer and the polishing pad, as well as from abrasive slurries. The surface of typical ceramic retaining rings may be relatively nonuniform, for example, having surface finishes of 6 to 8 microinches root mean square (xe2x80x9crmsxe2x80x9d). When subjected to the planarization process, ceramic particulates can be fractured from the nonuniform surface of the retaining ring. These ceramic particulates can cause scratches in the wafers that are being polished. Wafers that are scratched provide lower device yield and may be considered scrap, resulting in increased costs to the consumer. Further, the short lifetime of the retaining rings due to wear is significant in that the retaining rings are typically expensive consumable component parts of the CMP apparatus.
The inside diameter corners of the retaining ring at the anchor surface and polishing surface of the retaining ring are particularly susceptible to wearing and chipping. FIGS. 1A and 1B illustrate a conventional ceramic retaining ring 10 available in the prior art. Retaining ring 10 has a polishing surface 12, which contacts a polishing pad, an anchor surface 20, which contacts the wafer carrier, an inside diameter surface 16, and an annular groove 18 which is positioned along the inside diameter surface 16. Annular groove 18 has a depth xe2x80x9cAxe2x80x9d as measured from inside diameter surface 16 and is configured to receive a disposable liner (not shown). Retaining ring 10 also has at least one anchor bore 14 for receiving an anchor device, such as a screw or bolt, so that retaining ring 10 may be fixedly attached to a CMP wafer carrier. Although anchor bore 14 is shown in FIG. 1B as opening to anchor surface 20 to receive an anchor device previously inserted into a wafer carrier, anchor bore 14 may also be a through-hole which receives an anchor device for subsequent insertion into the wafer carrier to anchor the retaining ring thereto. A corner 22, which is the intersection of polishing surface 12 and inside diameter surface 16 of retaining ring 10, has a radius generally on the order of from approximately 0.002 inches to 0.005 inches. With a radius in this range, corner 22 is relatively xe2x80x9csquaredxe2x80x9d in shape. During the planarization process, polishing surface 12 and corner 22 are put in contact with a polishing pad (not shown) and subjected to rotational, lateral and/or orbital motion relative to the polishing pad. The inventors have discovered that forces on retaining ring 10 from this motion can cause chipping of corner 22, resulting in ceramic particulates separating from retaining ring 10.
Another problem with conventional ceramic retaining rings is that they can cause wear of bladder assemblies that are used in some wafer carriers to urge the wafer against the polishing pad with a desired amount of force. FIG. 2 depicts a conventional wafer carrier 100 that uses such a bladder assembly. For the sake of clarity and brevity, wafer carrier 100 is illustrated in a simplistic manner without a number of components that may be present in a practical carrier. Typically, carrier 100 is mounted at the end of a rotatable and vertically movable drive shaft 102, and above a rotatable polishing surface, e.g., a polishing pad 104, affixed to a platen 105. Wafer carrier 100 includes a pressure element 108, a retaining ring 10, and a flexible bladder membrane 106 which surrounds a cavity 110. Cavity 110 is in fluid communication with a gas or fluid source (not shown) which, when activated, fills cavity 110 with gas or fluid and causes flexible bladder membrane 106 to urge a wafer 112 against polishing pad 104. As bladder membrane 106 expands due to the air or fluid in cavity 110, it contacts a corner 24 of retaining ring 10. Referring momentarily back to FIG. 1B, corner 24 is formed from the intersection of anchor surface 20 and inside diameter surface 16 and has a radius also generally on the order of from approximately 0.0020 inches. With a radius in this range, corner 24 also is relatively xe2x80x9csquaredxe2x80x9d in shape. Consequently, bladder membrane 106 is worn by the friction caused by contact with corner 24. The inventors have discovered that such wear reduces the life of bladder membrane 106 and causes particulates from the bladder membrane 106 to separate from bladder membrane. These particulates can also result in scratches to wafer W.
Accordingly, there is a need for an apparatus for eliminating or reducing wear of workpiece carrier elements in order to optimize workpiece throughput rate and enhance uniform polishing of workpieces.
This summary of the invention section is intended to introduce the reader to aspects of the invention and is not a complete description of the invention. Particular aspects of the invention are pointed out in other sections hereinbelow, and the invention is set forth in the appended claims which alone demarcate its scope.
In accordance with an exemplary embodiment of the present invention, a wafer carrier retaining ring is provided. The wafer carrier retaining ring includes a first annular surface which contacts a polishing surface during a polishing process. The wafer carrier retaining ring also includes an inner diameter surface that adjoins the first annular surface thereby forming a first annular corner. The first annular corner has a radius in the range of from no less than about 0.010 inches to less than a radius that would result in damage to a wafer being polished.
In accordance with another exemplary embodiment of the present invention, a wafer carrier retaining ring has a first annular surface which contacts a polishing pad during a polishing process. The wafer carrier retaining ring is formed of a ceramic material and the first annular surface has a surface finish of no greater than about 2 microinches rms.
In accordance with a further embodiment of the invention, a wafer carrier retaining ring has a first annular surface which contacts a polishing pad during a polishing process. An inner diameter surface adjoins the first annular surface. The wafer carrier retaining ring includes a second annular surface that is positioned parallel to the first annular surface and that adjoins the inner diameter surface, thereby forming an annular corner. The annular corner has a radius of no less than 0.030 inches.
In accordance with yet another embodiment of the invention, a wafer carrier assembly is provided. The wafer carrier assembly includes a pressure element that is configured to press a wafer against a polishing surface. A retaining ring is mounted to the pressure element. The retaining ring comprises a first annular surface which contacts the polishing surface during a polishing process. An inner diameter surface adjoins the first annular surface thereby forming a first annular corner. The first annular corner has a radius in the range of from no less than about 0.010 inches to less than a radius that would result in damage to a wafer being polished.
In accordance with yet a further embodiment of the invention, a wafer carrier assembly has a pressure element configured to press a wafer against a polishing surface. A retaining ring is mounted to the pressure element. The retaining ring has an annular surface which contacts the polishing surface during a polishing process. The retaining ring is formed of a ceramic material and the annular surface has a surface finish no greater than about 2 microinches rms.